The present invention relates in general to continuous rotation scanning antennas for use in surveillance radars, and in particular to a scanning antenna configuration that does not require the use of a rotary joint.
The possibility of terrorist activity, compromise of military information, or material theft results in the need to protect various high value assets whether located in permanent or temporary sites. A desirable approach protecting such valuable assets is the establishment of a network of low power surveillance radars to provide automated perimeter security. For greatest versatility, the surveillance radars should be easily transportable and deployable in multiple emplacements in any desired positional configuration. Therefore, the surveillance radar should be small in size and have a weight low enough for single person installation.
A typical example of a multiple surveillance radar deployment is shown in FIG. 1. An aircraft parking area 1 containing high value assets, such as aircraft 2, is encompassed by a multiplicity of surveillance radars 3 spaced so that the detection volumes 4 provided by each radar form a continuous zone for the detection of intruders around the perimeter of the area 1. The cost of such an installation should be affordable, and thus each surveillance radar should be designed and constructed in a manner to minimize cost while providing the required performance. The surveillance radars should provide an azimuthal scan of 360 degrees to allow for versatility of placement, and a scan rate sufficiently high that an intruder cannot traverse the radar""s detection volume without being intercepted by a scan of the beam and thus detected. Operation in the millimeter wave region of the electromagnetic spectrum allows the use of a small, lightweight-scanning antenna that produces a narrow beam in azimuth for adequate resolution of target details.
The prior art employs various methods in the design of continuous rotation, 360-degree scan antenna systems, especially for microwave radars. To accomplish focusing of the transmitted beam, the antenna can employ either a parabolic reflecting element or a refracting element with a microwave feed located at the focal point of the element, a planar array made up of slotted waveguides, or equivalent electromagnetic structure, etc. One common technique for coupling the microwave signals from the antenna to the transmitter and receiver subelements is to place these subelements in the stationary portion of the radar. The transmitted and received signals are transferred to and from the antenna by a rotary joint placed upon the axis of rotation of the antenna.
Another technique is to locate much or all of the transmitter and receiver subelements with the antenna on the rotating structure, and transfer raw power and control signals from, and receiver output video to the stationary portion of the radar via slip rings coupled to the rotational axis. This technique has disadvantages of significant transmitter/receiver weight forming part of the rotating mass, a relatively uncontrolled environment for critical transmitter/receiver circuitry, and signal noise generated by the slip ring assembly.
The first described technique using a rotary joint is generally preferable. Rotary joints operating in the microwave region of the electromagnetic spectrum are widely used and provide adequate performance. However, those that operate in the millimeter wave region of the spectrum may not provide adequate performance and are prohibitively expensive for use in a low cost surveillance radar.
One example of prior art is the reference Waters et al., statutory invention reg. no. H966, published on Sep. 3, 1991. Waters provides a scanning antenna requiring no rotary joint for use in a shipboard environment. In the stationary portion of this design, the electromagnetic energy is collimated into a beam of significant diameter by means of a parabolic reflector. This beam is transmitted upward to a rotating assembly that by phase sensitive reflection produces two scanning, orthogonally polarized beans transmitted horizontally in opposite directions. The physical mechanism that supports the scanning assembly must provide unobstructed passage of the rather large diameter collimated beam from the stationary parabolic reflector to the rotating assembly.
In view of the above, there is a need for an improved method of transferring the millimeter wave electromagnetic energy between the rotating antenna and the transmitter and receiver subelements located in the stationary portion of the radar. Furthermore, there is a need to accomplish this without requiring the use of slip rings or a rotary joint, and by using a minimum of components in a lightweight configuration having reasonable cost. For these and other reasons, there is a need for the present invention.
The invention relates to a surveillance radar-scanning antenna requiring no rotary joint. The surveillance radar antenna of the invention includes a millimeter wave horn positioned on the vertical axis of rotation of the antenna and protruding through the open center of the antenna support bearing, driven gear, and a hole in the antenna housing. Divergent millimeter wave electromagnetic energy is emitted vertically by the non-rotating horn, then is deflected to the horizontal by an angled reflector before being focused by a dielectric lens into a collimated beam. The rotating antenna housing supports the angled reflector and dielectric lens. Provisions are made for vertical positioning of the dielectric lens to allow limited adjustment of the transmitted beam above or below the horizontal. Received energy reflected from distant targets is collected by the dielectric lens and directed by the angled reflector to the non-rotating horn where it is fed to a waveguide coupled to the receiver.
The present invention provides a method for the transfer of millimeter wave electromagnetic energy between a rotating antenna assembly and the transmitter and receiver subelements in the stationary structure of the radar. An advantage of the present invention is that a millimeter wave rotary joint, with its intrinsic requirement for extremely accurate tolerances and highly expensive manufacturing processes, is not required. Another advantage is that a surveillance radar incorporating the present invention does not have any moving mechanical parts in the waveguide portion of the electromagnetic energy path. Furthermore, the radar of the invention experiences no variation in energy loss due to variations in a mechanical rotary joint, and does not require the periodic replacement of an expensive rotary joint component.
In contrast to Waters, the present invention uses a support bearing and driven gear, which supports and drives the rotating antenna structure, with open inner diameters only sufficiently large to allow passage of a non-rotating millimeter wave waveguide assembly. The electromagnetic beam is emitted by a non-rotating horn and then collimated by an angled reflector and dielectric lens forming a part of the rotating portion of the antenna. Other aspects, embodiments, and advantages of the prior art will become apparent by reading the detailed description that follows, and by referring to the accompanying drawings.